This invention relates generally to extrusion compounding of thermoplastic resin with reinforcing fibers. More particularly, a multiple extruder apparatus and process for producing a preform suitable for subsequent use in a molding operation, such as compression molding, is disclosed.
Compounding of both thermoplastic and thermosetting resins with reinforcing fibers in an extruder is known in the prior art. However, the prior art compounding apparatus and processes present problems and disadvantages which are overcome by the particular, multiple extruder system disclosed herein.
In the known prior art compounding procedures, the fibers and resins are normally introduced together into a compounding extruder in a solid state. In such a compounding operation, the fibers are broken down into very short lengths, usually less than 0.10 inches, due to both the mechanical action of the extruder screw on the fibers, and the presence of thermoplastic resins in a high viscosity state when the resin begins to melt. The fibers are thus present in the extruder when the thermoplastic resin is being initially transformed from a solid to a liquid state. Upon first being converted from a solid to a liquid by a combination of mechanical working and heat, thermoplastic resins have a relatively high viscosity, which contributes to fiber breakage as the resins are intermixed with the fibers in the extruder. U.S. Pat. No. 4,422,922 discloses such a compounding extruder apparatus and process. According to the disclosure of that patent, a single, twin screw extruder is utilized for both initially melting the polymer and for blending or compounding carbon fibers with the melted polymer. In a first disclosed procedure, the polymer and reinforcing fibers are both introduced into the extruder together at a supply hopper. In a second embodiment of that patent, a thermoplastic resin polymer is introduced into a first extruder zone where it is melted, and from which it is fed into a second extruder zone where the fibers are introduced. This procedure has the disadvantage that the fibers are introduced downstream of the thermoplastic resin input point in the compounding extruder. Thus, relatively cold fibers draw heat out of the melted thermoplastic resin, thereby raising the viscosity of the resin. The resulting, somewhat more viscous thermoplastic resin contributes to fiber breakage during the extruding process. This procedure has further disadvantages. The screw speed desirable for melting the resin is not necessarily the screw speed which would be most effective for compounding resin and fibers. Also, the relative volumes of the resin and fibers must be closely controlled in order to achieve the desired composite product; and it is difficult to properly match the volumes of those two components when compounding in a single screw of specific geometry.
My U.S. Pat. Nos. Re. 32,772 and 4,312,917 disclose a method and apparatus respectively for making a composite consisting of reinforcing fibers embedded in a thermoplastic resin material. The fiber-reinforced composite is formed by passing extruded plastic resin through a stationary die into which continuous lengths of reinforcing fibers are introduced in the presence of the heated, molten thermoplastic resin. The continuous length fibers are pulled through the die, in which they pass over lobes and are impregnated with the molten thermoplastic resin to form an extruded, plastic member having a predetermined shape with fiber strands extending continuously, and longitudinally therein. The extruded plastic member may be cut into short, pellet-size lengths for use as a molding compound.
U.S. Pat. Nos. 4,393,020 and 4,500,595 disclose methods for manufacturing a fiber-reinforced thermoplastic composite. However, both patents utilize conventional, batch processes for carrying out the step of forming the polymer-fiber composite wherein the fibers are drawn through or dipped in a bath of the molten, thermoplastic resin. In U.S. Pat. No. 4,393,020 a process is disclosed in which the fibers are preferably oriented in one direction in the resin-fiber composite, and in both patents, the composite may have relatively long fibers, and is used to mold end products, as by injection molding.
Compression molding has certain advantages over injection molding, one of which is the capability of producing molded parts with significantly longer reinforcing fibers. The presence of the long reinforcing fibers creates plastic parts with improved mechanical properties. The plastic raw materials utilized in compression molding vary from sheet to bulk molding compounds (a puttylike mixture) to pellets. The bulk molding compound (BMC) and the pellet products are usually first converted to a preform prior to molding, but not always. The preform may be any desired shape, such as a sheet or "log" type of member which is introduced into the chamber of the compression mold. The molding compounds are made up of at least two components, including a reinforcing fiber, such as glass or carbon, and a plastic resin. Various fillers and additives, as well as colorants may be utilized in the molding compound. The plastic resin can be either thermosetting (requiring heat and pressure to increase molecular weight to form a solid substance), or thermoplastic (high molecular weight resins that require heat for melting and cold for solidifying). The vast majority of the compression molded parts today are made with thermosetting resins. Thermoplastic resins have some significant advantages over thermosetting resins when used in the compression molding process. Faster molding cycles and greater toughness in the molded articles are two of them. Thermoplastics have only recently been applied to compression molding. Raw material suppliers offer thermoplastic products for compression molding in two forms, sheets and pellets.
Thermosetting bulk molding compounds (TS-BMC) are in the form of a puttylike substance. The product is divided into chunks suitable in size for the intended application. This can be done by hand or in a commercially available machine that automatically produces chunks (preforms) of the correct size. The preform is placed into an open mold heated to the desired temperature. The resin melts, flows, chemically chain extends and cross links to a solid. The mold is opened and the part is removed so that the cycle is ready to be repeated. The mold is normally heated to approximately 300.degree.-350.degree. F. Thermosetting pellets, granules, or powder can be weighed and poured into an open mold heated to approximately the same temperature.
The foregoing molding methods show the inherent disadvantages of compression molding with a thermosetting resin product. In the mold, the resin must first melt, then flow, and then increase in molecular weight to a point where the product is a solid at the mold temperature. The result is a long cycle time. This long cycle time is costly to both the molder and the end user. This inherent disadvantage makes molding with the use of thermoplastics particularly attractive. Thermoplastics need only be cooled in the mold, resulting in substantial increases in productivity.
Thermoplastic sheet molding compounds (TP-SMC) are cut to sizes suitable for the intended application. The sheet is placed in an oven, the resin is melted, and the melted sheet is placed in an open mold heated to approximately 100.degree.-200.degree. F. This temperature is significantly lower than the melting point of the thermoplastic resin. The mold is closed rapidly, causing the resin to flow, filling the mold cavity. The relatively cold temperature of the mold causes the resin to harden rapidly. The mold is opened and the part is removed, allowing the mold to be recycled. Two limitations as to this method include inconsistency in the sheet product, and the inability to modify the composite formulation.
The foregoing limitations are eliminated in thermoplastic bulk molding compounds (TP-BMC). This product is available in pellet form. The pellets are fed into a commercially available machine that melts the resin, mixes the product uniformly, and produces a precisely measured preform. This hot preform is then placed in the mold, pressed, cooled, and removed. The part-to-part consistency is controlled and additives can be added that will alter the formulation of the composite, as needed. This TP-BMC process is the newest form of compression molding. Although it eliminates the limitations of TP-SMC, it also creates several new process difficulties.
First of all, in order to produce high physical properties in the molded article, long reinforcing fibers must be present. These long fibers are produced by cutting the TP-BMC pellet to lengths of one to two inches, typically. These long pellets are hard, rigid rods that do not handle particularly well in automated equipment. As the long pellet enters the preformer, it is often cut by the extruder screw flight as it passes the feed opening, thereby reducing the desired fiber length. Secondly, the long pellets cannot be dried in conventional dryers. Only resins that are nonhygroscopic can be molded at this time in the TP-BMC process. Some hygroscopic resins would make excellent finished parts in compression molding. Further disadvantages are that the molder must pay the cost of compounding the long fiber product into pellet form, as purchased by the molder, and multiple heat histories on the molding pellets reduce the physical properties of the finished part.
The foregoing problems and difficulties associated with prior art compression molding techniques would be eliminated if the molder could compound the thermoplastic molding product in-house in specially designed equipment that is connected to the preform-making equipment. The concept of in-house compounding by the molder is not new. However, the procedure is utilized primarily in the compression molding of thermosetting plastics, i.e., TS-BMC. Such compounding is done in mixers unsuitable for thermoplastics. Some thermoplastic injection molders also compound short fiber pellets in-house.
Accordingly, this invention is directed to apparatus and process for compounding long fiber (one inch in length or greater) with a thermoplastic resin in an extrusion process. The molder carries out this procedure, after which the hot mixture is fed directly into the preform-making equipment and from there, directly into the compression mold, if desired.